ChemistryICSE Class 8 · Water

Water of Crystallisation, Hydrated and Anhydrous Salts and Hygroscopic Substances (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a crystalline salt, which gives it shape and colour. Hydrated salts contain it (blue CuSO₄·5H₂O); anhydrous ones do not (white CuSO₄). Hygroscopic substances absorb moisture from air without dissolving and are used as drying agents.

Heating drives out the 5 water molecules so blue turns white; adding water restores the blue, so white CuSO₄ is a test for water.

Illustration drawn for QuestionBuddy.

Understand Water of Crystallisation, Hydrated and Hygroscopic Substances

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a crystalline salt. It gives the crystal its shape and often its colour.

Hydrated substance: contains water of crystallisation, e.g. blue copper sulphate $C u S O_{4} \cdot 5 H_{2} O$ .
Anhydrous substance: no water of crystallisation, e.g. white copper sulphate $C u S O_{4}$ .

Common name	Formula
Blue vitriol (copper sulphate)	CuSO₄·5H₂O
Green vitriol (iron(II) sulphate)	FeSO₄·7H₂O
White vitriol (zinc sulphate)	ZnSO₄·7H₂O
Washing soda (sodium carbonate)	Na₂CO₃·10H₂O
Epsom salt (magnesium sulphate)	MgSO₄·7H₂O
Gypsum (calcium sulphate)	CaSO₄·2H₂O

Heating blue vitriol. The blue crystals lose their water and become a white powder:

$C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$

Adding a few drops of water turns the white powder blue again and it gets warm. So anhydrous copper sulphate is a test for water.

Hygroscopic substances absorb moisture from the air without dissolving in it. Examples: concentrated sulphuric acid, quicklime (calcium oxide), silica gel. They are used as drying agents, e.g. silica gel sachets in shoe boxes and medicine bottles.

Distinguish. Water of crystallisation is *inside* the crystal in a fixed number; a hygroscopic substance takes water *from the air*. A hydrated salt has already taken its water and is not a drying agent.

Counting atoms. In $C u S O_{4} \cdot 5 H_{2} O$ : Cu 1, S 1, O $4 + 5 = 9$ , H $5 \times 2 = 10$ .

Water of Crystallisation, Hydrated and Hygroscopic Substances: formulas and key facts

Blue vitriol: $C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$; Blue to white on heating; white to blue on adding water.
Common hydrates: Blue vitriol CuSO₄·5H₂O
Green vitriol FeSO₄·7H₂O
White vitriol ZnSO₄·7H₂O
Washing soda Na₂CO₃·10H₂O
Epsom salt MgSO₄·7H₂O
Gypsum CaSO₄·2H₂O; Number of water molecules after the dot.
Hygroscopic: Conc. H₂SO₄
Quicklime
Silica gel; Absorb moisture without dissolving.

How to solve Water of Crystallisation, Hydrated and Hygroscopic Substances questions

Read the number after the dot as the water molecules per formula unit and use it to count atoms.

1The number in front of H₂O is the number of water molecules.
2Multiply H₂O (2 H, 1 O) by that number and add to the salt's atoms.
3For a mass loss on heating, water lost = mass before − mass after; percentage = water lost ÷ mass before × 100.
4A colour change blue to white on heating shows loss of water of crystallisation.

Water of Crystallisation, Hydrated and Hygroscopic Substances in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Silica gel in a shoe box

Suppose a new pair of shoes comes with a small packet of silica gel.

Why is it included?

Try it first, then show the solutionSolution

Silica gel is hygroscopic.
It absorbs moisture from the air in the box, keeping the shoes dry.

Answer: It acts as a drying agent.

Memory hook: Hygroscopic: moisture magnet.

Is the sample pure water?

Imagine a lab technician adds a colourless liquid to a pinch of white anhydrous copper sulphate and the powder turns blue.

What does this show?

Try it first, then show the solutionSolution

The white anhydrous salt combined with water to form blue hydrated copper sulphate.
So the liquid contains water.

Answer: The liquid contains water.

Memory hook: White to blue: water present.

Tricks to remember Water of Crystallisation, Hydrated and Hygroscopic Substances

Short memory aids. Say each one aloud once, then use it on a question the same day.

Blue = wet, white = dry
CuSO₄·5H₂O is blue; CuSO₄ is white.
Use it when: Test for water.
Dot means joined
The dot joins the salt to its water molecules.
Use it when: Reading formulae.
Hygroscopic does not dissolve
It absorbs moisture from air without dissolving.
Use it when: Definition.

Water of Crystallisation, Hydrated and Hygroscopic Substances: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Formula of blue vitriol · 1 mark · MCQBoard level

The formula of blue vitriol is:
(a) CuSO₄
(b) CuSO₄·5H₂O
(c) FeSO₄·7H₂O
(d) Na₂CO₃·10H₂O

Try it first, then show the solutionSolution

Blue vitriol is hydrated copper sulphate with 5 water molecules.

Answer: (b)

Example 2: Heating and adding water · 2 marks · Short answerBoard level

Blue crystals of copper sulphate are heated, and then a few drops of water are added to the residue. State the observations in each step and write the equation for the first step.

Try it first, then show the solutionSolution

Heating: blue crystals turn to a white powder and water vapour is given out. $C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$ . (1)
Adding water: the white powder turns blue and warms up. $C u S O_{4} + 5 H_{2} O \to C u S O_{4} \cdot 5 H_{2} O$ . (1)

Answer: Blue → white; white → blue.

Example 3: Counting atoms · 3 marks · Short answerHarder board level

For one formula unit of (a) $C u S O_{4} \cdot 5 H_{2} O$ , (b) $N a_{2} C O_{3} \cdot 10 H_{2} O$ find the number of oxygen atoms and the number of hydrogen atoms.

Try it first, then show the solutionSolution

(a) O $= 4 + 5 = 9$ ; H $= 5 \times 2 = 10$ . (1½)
(b) O $= 3 + 10 = 13$ ; H $= 10 \times 2 = 20$ . (1½)

Answer: (a) O 9, H 10 (b) O 13, H 20

Example 4: Loss of mass · 4 marks · Long answerHarder board level

5.0 g of blue copper sulphate crystals leave 3.2 g of white powder on heating. (a) Find the mass of water lost. (b) Find the percentage of water in the crystals. (c) How much water is contained in 25 g of the crystals?

Try it first, then show the solutionSolution

(a) $5.0 - 3.2 = 1.8$ g. (1)
(b) $\frac{1.8}{5.0} \times 100 = 36%$ . (1½)
(c) $25 \times 0.36 = 9$ g. (1½)

Answer: (a) 1.8 g (b) 36% (c) 9 g

Example 5: Silica gel and blue vitriol · 4 marks · Case-basedHOTS / competency

A sachet of silica gel is kept in a medicine bottle. (a) Which property of silica gel is used? (b) Would crystals of blue vitriol do the same job? Explain. (c) Name two other hygroscopic substances. (d) How would you show that a liquid contains water?

Try it first, then show the solutionSolution

(a) It is hygroscopic: it absorbs moisture from the air. (1)
(b) No; blue vitriol is already hydrated and holds its water of crystallisation, so it is not a drying agent. (1)
(c) Concentrated sulphuric acid and quicklime. (1)
(d) Add it to white anhydrous copper sulphate: it turns blue. (1)

Answer: (a) hygroscopic (b) no (c) conc. H₂SO₄, quicklime (d) anhydrous CuSO₄ turns blue

Where students lose marks in Water of Crystallisation, Hydrated and Hygroscopic Substances

The slips examiners see most often, and what to write instead.

Mistake: Confusing hygroscopic with hydrated
Hydrated: water in the crystal. Hygroscopic: absorbs from air.
Wrong working
"Silica gel is a hydrated salt."
Right working
"Silica gel is hygroscopic."
Do this: Say where the water is.
Mistake: Forgetting to multiply
Water has 2 H and 1 O.
Wrong working
"CuSO₄·5H₂O has 5 H."
Right working
"It has 10 H."
Do this: Multiply by the number after the dot.
Mistake: Wrong colour change
Heating makes it white.
Wrong working
"Anhydrous copper sulphate is blue."
Right working
"Anhydrous is white; hydrated is blue."
Do this: Blue = water.

Exam tips

Write the formula with the dot correctly.
Quote observations with colour for the heating test.
Give the use of anhydrous copper sulphate as a test for water.
Name two hygroscopic substances.

Practice questions on Water of Crystallisation, Hydrated and Hygroscopic Substances

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
What is meant by a hygroscopic substance? Give one example.
Hint
Absorbs moisture.
Answer
A substance that absorbs moisture from the air without dissolving in it, e.g. silica gel.
Q2 · 2 marks · Short answerHarder board level
Why do blue copper sulphate crystals lose their colour and shape on heating?
Hint
Think of the crystal water.
Answer
Water of crystallisation gives the crystal its blue colour and its shape. On heating it is driven off, leaving white anhydrous copper sulphate in a powdery form.
Q3 · 3 marks · Short answerHarder board level
Give the formulae of green vitriol, white vitriol and Epsom salt and state the number of water molecules in each.
Hint
Table.
Answer
Green vitriol FeSO₄·7H₂O (7), white vitriol ZnSO₄·7H₂O (7), Epsom salt MgSO₄·7H₂O (7).
Q4 · 3 marks · Short answerHarder board level
Distinguish between a hydrated salt and a hygroscopic substance.
Hint
Inside or from air.
Answer
A hydrated salt has a fixed number of water molecules chemically combined within the crystal. A hygroscopic substance takes moisture from the air without dissolving and can be used as a drying agent.
Q5 · 4 marks · Long answerHOTS / competency
The white powder obtained by heating blue vitriol is exposed to damp air and slowly turns pale blue. Explain, and say whether it is hygroscopic.
Hint
Think of the reverse reaction.
Answer
The anhydrous copper sulphate absorbs moisture from damp air and recombines with it to form blue hydrated copper sulphate (CuSO₄ + 5H₂O → CuSO₄·5H₂O). So it does absorb moisture from the air and can be used as a drying agent, but it does so by chemical combination to form a hydrate.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Formula of blue vitriol?Show answer
CuSO₄·5H₂O.
2.Colour of anhydrous CuSO₄?Show answer
White.
3.A hygroscopic example?Show answer
Silica gel.
4.Washing soda formula?Show answer
Na₂CO₃·10H₂O.
5.Test for water?Show answer
White anhydrous CuSO₄ turns blue.

Water of Crystallisation, Hydrated and Hygroscopic Substances: frequently asked questions

What is water of crystallisation?

It is the fixed number of water molecules chemically combined in one formula unit of a crystalline salt, giving the crystal its shape and often its colour.

How do you test for water?

Add the liquid to white anhydrous copper sulphate; it turns blue (and warms) if water is present.

What are hygroscopic substances used for?

As drying agents, such as silica gel in shoe boxes and medicine bottles.